The present invention relates to an electronic drive control apparatus for controlling a drive coil of a contactor.
An electronic drive control for a magnetic drive featuring pulse width modulation of the armature current is known from European Patent Application No. EP 0 789 378 A1. The armature coil is operated with a constant voltage. U.S. Pat. No. A-5,579,194 describes an electronic drive control for controlling a drive coil of a contactor, the electronic drive control having a pulse width signal which is generated by an arithmetic unit and having a semiconductor switch. In this context, the pulse width ratio can be varied by an impulse-shaping stage connected downstream.
It is also known to use microcontrollers for control systems of that kind. During the so-called xe2x80x9cholding modexe2x80x9d, the pick-up current is reduced by the factor 7 to 12. To keep the holding current and, consequently, the power loss in the drive coil as low as possible, it is required for the holding current not only to be reduced but also to be maintained constant at as low a value as possible over the entire voltage range because of the large voltage range for which such control systems are intended to be usable. The frequency of a pulse width modulator (PWM) and the TON-time, respectively, cannot be selected arbitrarily. Because of noise generation of the magnetic circuit due to the pulsing of the drive coil, unlike the pick-up control during which this short-time noise generation is completely covered by the moving process of the entire drive, it is required for the PWM frequency for the holding mode to be fixed to a frequency which lies outside the human hearing range. Frequencies outside the human hearing range lie above 20 kHz, which corresponds to a period duration of 50 xcexcs. Due to the required current reduction to the holding mode, a relatively short TON-time of, for example, 400 ns would ensue, making allowance for control, or actuating, reserves and other factors.
However, this cannot always be achieved since, when using microcontrollers, the pulse ratio of the PWM modulator cannot be adjusted arbitrarily but only as a whole-number multiple of the pulse frequency or of a variable derived therefrom. When working with controllers of that kind, oscillator frequencies of, for example, 10 MHz are usual, the oscillator frequencies being internally reduced to a pulse frequency of 1 MHz. Therefore, only a minimum of 1 xcexcs can be adjusted as the shortest TON-time.
However, considerably shorter TON-times are necessary to avoid noises which are unpleasant for the human ear and to be able to operate such a drive in the holding mode in a manner that saves as much energy as possible, involving as low a loss as possible. Moreover, it has to be taken into account that drives of that kind are intended to be suitable for a relatively large voltage range and that the holding pulses need to be correspondingly variable.
Therefore, an object of the present invention is to provide an electronic drive control apparatus for controlling a drive coil of a contactor which allows the holding mode to be implemented over a large voltage range with little loss and noise.
The present invention provides an electronic drive control apparatus for controlling a drive coil of a contactor. The electronic drive control apparatus includes a microcontroller configured for generating an inverted pulse width modulated signal; at least one semiconductor switch; and a pulse-shaping stage connected downstream of the microcontroller, the pulse-shaping stage being capable of varying the pulse width modulated signal. The pulse-shaping stage includes a preselectable RC combination and an inverter connected downstream of the RC combination and having a Schmitt trigger input. The inverted pulse width modulated signal is switched to the preselectable RC combination and the semiconductor switch is controllable by the inverter. A preselection of the RC combination is performed using a plurality of outputs of the microcontroller to open collector transistor stages of discharge resistors of the RC combination. Moreover, a pulse width of a driving signal of the semiconductor switch is determined from the inverted pulse width modulated signal and the preselected RC combination.
The present invention allows the holding pulses to be correspondingly finely stepped, or graded. The holding power is minimized in a simple manner.